As is known, aircraft in general must be guaranteed enough power to run the on-board electric equipment and user devices, as well as to power the main propulsion system, at least at the start-up stage.
This is currently achieved using so-called reversible electric machines, i.e. which operate as a generator when powered by the main propulsion system, and as an electric motor when powered by auxiliary electric power devices.
Currently used electric machines, which are normally variable-frequency alternating-voltage types, are still conceived as “independent units or modules”, and are usually installed outside a gear housing, in which one or more gear transmissions are housed inside a fixed casing. The gear housing is normally connected to the main propulsion system by one or more transmission shafts and one or more gear transmissions, also assembled inside fixed casings.
The electric machine has its own input/output shaft connected to an input/output shaft of the gear housing and fitted with the electric machine rotor, the stator of which is connected to a dedicated supporting frame of the electric machine.
The location of the electric machine with respect to the gear housing allows maintenance and replacement of the electric machine, even with the propulsion system assembled on the aircraft.
Modern aircraft need increasingly more electric power to cater to the increase in sophisticated on-board actuating and control systems, as well as to power electric equipment now replacing traditional hydraulic and/or pneumatic equipment.
The increase in electric power demand means an increase in the size and weight of the electric machine and therefore of the gear housing as a whole, thus making it difficult to accommodate the electric machine/mechanical transmission assembly in the existing space in the propulsion or power generating systems.